This invention relates in general to circuit interrupters and more particularly to fluid-blast circuit interrupters of the puffer type.
Puffer interrupters have enjoyed commercial success due in part to their simple construction and excellent service record. The increased use of puffer interrupters in power class circuit breakers has been at the expense of the more complex two-pressure interrupters. Unfortunately, compared to a two-pressure interrupter, a puffer interrupter requires a relatively large prime mover.
The prime mover of a power class interrupter is a small part of the total cost of the interrupter relative to the cost of a prime mover in a distribution class or subtransmission class breaker or interrupter. Therefore, the cost of a large prime mover has less impact on the total cost of a power class circuit breaker than on the cost of a distribution class breaker. If one were to design a cost effective distribution class breaker or recloser using the puffer interrupter concept, the designer must minimize the energy consumed by the prime mover in operating the breaker.
The breaking process is characterized by an arc appearing for a limited period of time across the gap between the opening contacts of the breaker. This arc plasma column imposes severe environmental conditions on the components of the interrupter. For example, the arc plasma has a temperature exceeding 20,000.degree. Kelvin; the turbulent supersonic flow of the quenching gas in a changing flow geometry ranges from a few hundred meters per second to several thousand meters per second; and, the voltage gradient placed upon the components in the vicinity of the arc is large, e.g. 10 KV/cm.
Immediately after the current passes zero, the critical stress is dependent upon the rate at which the recovery voltage rises. This rate is relatively high following a short-line fault, e.g. 2-7 KV/usec. Those points at which the arc roots are located after contact separation are particularly high stressed and in addition are contaminated with metallic vapor and ionized fluid. Typically, these arc by-products are deionized and removed by a concentrated gas blast in the vicinity of the arc. Because the gap region is open to the interior surface of the interrupter housing, part of the arc by-products are dissipated to the surrounding gas, while the majority is drawn through the tubes or hollow contact elements of the puffer.
Normally the nozzle surfaces across which the arc is formed "ablate" during the interruption process. As a nozzle ablates, its dimensions change and the arc geometry is effected. Ultimately, with the deterioration of the nozzle surfaces, the interruption rating of the device will be effected. Thus, the service life of a puffer interrupter will be increased by having components in the vicinity of the arc that effect the geometry or the flow of arc extinguishing fluid resistant to ablation by the arc.
In addition to the ablation problem, those skilled in the art have known that materials in the gap having a dielectric constant different from that of the gas or arc extinguishing fluid cause a distortion in the surrounding potential field. This distortion can cause high-voltage stresses to appear across the gap. These high-voltage stresses, in turn, can initiate a flashover of the contacts while they are separated or when the interrupter is opened.
If the materials having a different dielectric constant, i.e., so called "shunting dielectrics", are removed, a more uniform potential field is created in the vicinity of the gap. This in turn would reduce the voltage stress and decrease the possibility of a flashover. In this context "shunting dielectrics" include such materials as Teflon, polytetrafluoroethylene, which have a dielectric constant significantly different from that of the quenching gas typically, sulphurhexafluoride (SF.sub.6).
Areas of low dielectric strength can also be reduced by increasing the circulation of gas within the interrupter. Increased gas circulation would prevent stagnant gas, the gas most recently involved in the interruption process, from remaining concentrated in any one particular area of the interrupter. Circulation would provide a "mixing action" which would insure that the arc extinguishing gas has a more uniform dielectric strength throughout the interior of the interrupter. The gas most recently involved in the interruption process has a lower overall dielectric strength. Therefore, it is important to insure that gas having a relatively low dielectric strength does not build up in regions such as that surrounding the open contact gap and any point where a significant voltage stress exists between the current carrying parts of the interrupter and the ground. Critical insulating surfaces are those insulating surfaces in close proximity to the arc blast. Thus, if these surfaces are shielded from the arc, the fall out of arc products is minimized and the surrounding insulating surfaces can withstand the high-intensity radiation from the arc without the danger of a restrike or flashover.
As the operating voltage and current at which a puffer interrupter recloser operates at is reduced, the size and manufacturing costs of the unit becomes more critical. While other inventors have recognized the problems associated with the interruption process, few have proposed an apparatus that efficiently and economically resolves the problem of voltage stress and gas circulation. Millianowicz, U.S. Pat. No. 3,946,183, employs puffer piston arcing contacts and current carrying contacts, all requiring a fairly large driver mechanism. Holmgren, et al, U.S. Pat. No. 4,489,226, discloses a puffer interrupter which employs a plenum rather than a nozzle for directing the flow of dielectric fluid which improves circulation, however, this puffer interrupter employs separate current carrying and arcing contacts which also requires a relatively large driver.
For the most part, each of these earlier designs is complicated, requires extensive mechanical linkages and connections, and is expensive to manufacture and maintain in operation once it is put in use. An inexpensive, relatively simple, innovative design for a small compact puffer interrupter of the subtransmission or distribution class variety would be a welcome addition to the art. This would be particularly true if the design incorporated features which would reduce maintenance and operating costs.